A number of methods have been developed for the continuous cooking of wood chips in order to improve in different ways the quality of the pulp with respect to, for example, tear strength, beatability, tensile strength, etc. Many of these methods have focused in different ways on controlling the concentration of alkali in the digester in order in this way to influence the process of delignification. It has in addition been discovered that in order to obtain an even pulp quality it is of great importance that the alkali profile across the cross-section of the digester is maintained as even as possible, and that the alkali profile is even and is not too high during the various phases of the cook.
Various suggestions for the adjustment of the alkali during the cook have been used in order to even out the alkali profile during the cook. It is possible, for example, to use adjustment flows, in which a volume of cooking fluid is withdrawn from the digester and returned to the digester after adjustment of the alkali, or where withdrawn cooking fluid that is returned to the digester is replaced fully or partially by dilution fluid, which primarily gives a reduction in what are known as “DOMs” (an acronym for “dissolved organic materials”), where DOM is principally constituted by hemicellulose and lignin, but also of cellulose and other extracted substances from the wood chips. The withdrawal of cooking fluid at several positions and the subsequent replacement of the withdrawn cooking fluid by another fluid, however, involves a reduction in the yield, since residual fibres and hemicellulose disappear with the withdrawn cooking fluid.
FIG. 1 shows different cooking technologies for continuous digesters introduced during the years 1957, 1962, 1983, 1991, 1993, 1997 together with a later patented variant known as “Xylan”. Strainer sections are shown as dashed sections with withdrawals to recovery plants/REC (a broad arrow with a solid arrowhead), or in the form of digester flows/Circ, in which fluid is recirculated back to the centre of the digester through conventional central pipes. Heat exchangers/HE are present in certain flow lines. CLiqFLOW indicates the direction of flow of the cooking fluid in the digester. The addition of white liquor is shown by WL. In addition to the flows that are shown, there is also, naturally, the addition of dilution fluid at the bottom of the digester, and a top separator in the input to the digester. TC denotes a first digester circulation with heating in which the addition of white liquor can be carried out in addition to that which is added before the digester or at the top of the digester. WC denotes the lower washing flow in which it is heated to a high washing temperature (typically to 120-130° C.) in the systems.
In the very early technology for continuous digesters, all cooking chemicals were added in batches before the cook or at its top, and the cooking fluid was present during the major part of the cook, to be finally withdrawn from the digester through withdrawal strainers arranged at the bottom. This technology was primarily intended for small digesters with a production capacity of a few hundred tonnes of pulp per day, where the digester had a limited diameter of the order of a maximum of 3-4 meters. It was still possible in these small digesters to withdraw sufficiently large volumes of spent cooking fluid from the digester, since it was only 1.5-2 meters in to the centre of the column of chips within the digester, and the speed of the chips was low due to the low production. This type of cooking process is shown schematically in FIG. 1; -57. The technology in which the digester did not have several strainer sections in the digester, and had only a single strainer section at the bottom, was used also for the cooking of finely chipped slivers, sawdust and one-year plants, where the original material was so finely divided that it was difficult to carry out the withdrawal from the digester since the chipped material was so tightly packed.
In conventional digester technology established during the period 1960-1970 for larger continuous digesters with production capacities of approximately 1000 tonnes of pulp per day, essentially all alkali was added batchwise at the top of the digester with highly located withdrawal strainers in the digester, where a withdrawal REC of spent cooking fluid for the recovery process took place at a time at which the chips had had a retention time of approximately 50-70% of the total retention time in the digester. It was conventional that a zone of countercurrent cooking and washing flow was established under this withdrawal strainer where washing fluid introduced at the bottom was drawn opposite to the flow of the sinking chips. The fluid in this countercurrent cooking and washing zone lies essentially well under the cooking temperature during the principal part of the retention time of the chips in this zone of countercurrent flow. It was normal that there was also a heating flow lowest down in this zone of countercurrent flow (at the beginning of the countercurrent flow) in which the fluid was heated to a temperature that typically lay 10-30° C. under the established cooking temperature in the superior cooking zone. This type of countercurrent washing zone is known also as “Hi-Heat” washing. This type of cooking process is shown schematically in FIG. 1; -62.
The modified continuous cooking technology, MCC, was introduced during the 1980s, as higher requirements for the quality of pulp were desired. The MCC technology means that the alkali is divided into several batches and it is typical that also a small batch of white liquor/WL was added in a flow to even it out arranged under the withdrawal strainer and in the zone of countercurrent flow. It was possible in this manner to obtain a certain evening out of the alkali profile in the cook, and a larger part of the digester was actively used as a cooking zone with an effective level of alkali, which allowed longer cooking times and lower cooking temperatures, which gave better pulp quality, and higher production capacity.
This type of cooking process is shown schematically in FIG. 1; -83 where the MCC flow has been indicated.
A further method to improve the quality of the pulp was developed with the ITC (an abbreviation for “Iso Thermal Cooking”) technology, where the highest cooking temperature and the alkali level were reduced relative to the prior art and were maintained at constant levels throughout the cook. This technology meant that washing fluid and cooking fluid added at the bottom of the digester were withdrawn in an extra strainer section and were warmed to full cooking temperature before return to the digester. The time during which the chips were held at full cooking temperature was extended in this manner to be valid for essentially the complete zone of countercurrent flow under the withdrawal section that withdrew spent cooking fluid to the recovery process. This type of cooking process is shown schematically in FIG. 1; -91 where the ITC flow has been indicated.
Very high fluid/wood ratios have begun to be used in pre-impregnation vessels and in the cooking zones of the digester with the aim of further evening out the alkali profile during the cook. This technology constitutes one of the bases of the COMPACT COOKING™ concept developed by Kvaerner Pulping. The alkali concentration in the cooking fluid can in this way be reduced while at the same time the amount of alkali needed for an effective neutralisation process remains in the cooking fluid. Since the fluid fraction per measure of chips is considerably raised, typically with a fluid/wood ratio that lies well over 3.0, it remains possible to guarantee that a sufficient amount of alkali, measured as a quantity of kilograms of alkali per kilogram of wood, is present for the de-lignification process, while the concentration of alkali at the same time does not need to be so high. As the production is raised to levels greater than 1,800-2,000 tonnes of pulp per day, also the position of the final cooking fluid withdrawal is displaced downwards in the digester, often in combination with several withdrawal positions during the cook. This type of cooking process is shown schematically in FIG. 1; -93, where two withdrawal positions are indicated, and in FIG. 1, -97, where three withdrawal positions are indicated.
Production capacities of 2,500-3,500 tonnes of pulp per day are required in the continuous digesters for chips that are installed now. These continuous digesters are very large with digester diameters of 8-10 meters, and occasionally even larger—around 12 meters in diameter. The problem of implementing withdrawal sections is exacerbated in these digesters, since it becomes more difficult to withdraw cooking fluid from the centre of the column of chips with these strainer sections. The withdrawal sections rapidly reach a limit for the volume of cooking fluid that it is possible to withdraw. One desire, therefore, is to limit the number of strainer sections and to retain the cooking fluid as far as is possible in the digester with a high fluid/wood ratio, according to the COMPACT COOKING™ concept.
It is also known that the yield of pulp is improved by the addition of additives of polysulphide type, as is shown in, for example, U.S. Pat. No. 6,241,851 and U.S. Pat. No. 6,569,851. The effective alkali concentration and the temperature conditions in the first treatment zone are such that essentially no alkali breakdown of the cellulose takes place: instead the material is effectively penetrated by the polysulphide. The material is subsequently treated with an alkali cooking liquor at the cooking temperature in order to produce a chemical cellulose pulp with higher yield from the cooking process than would be achieved if pre-treatment at low temperature, low alkali and in the absence of polysulphide.
Through SE 520 956 is known a method to increase the quality of the pulp with respect to pulp strength, bleachability and reduced subsequent yellowing, while the yield over the digester increases at the same time. This is possible in that all withdrawal fluids, and in particular the hemicellulose-rich impregnation fluid, are allowed an extended retention time outside of the digester before this is returned to the same zone or the immediately subsequent zone. This means that the H factor of the cooking and impregnation fluids increases, i.e. it means that this cooking fluid is given a more extended retention time at the cooking temperature than the retention time that the chips are given. The principle is that a long time is required before the hemicellulose starts to precipitate onto the fibres, which is a process that occasionally takes a retention time for the hemicellulose-rich cooking fluid longer than 60 minutes. It is possible with this technology simply to extend the retention time of the cooking fluid in the system such that this precipitation process can be initiated, something that is appropriate for the cooking systems that do not have sufficient time to activate the precipitation process with the relevant type of wood. It is the intention that as much hemicellulose as possible will be given the opportunity to have time to precipitate onto the fibre, which gives an increased yield of fibre and in certain cases an increase in its strength properties.
It has, however, proved to be the case that the method in SE 520 956 does not give the intended increase of the strength properties of the pulp fibre in all cooking systems or for all types of wood. By increasing the H factor of the impregnation liquor through a retention time that increases the time, it will indeed be the case that more of the hemicellulose that is dissolved from the chips will re-precipitate onto the pulp fibre, but the strength-raising properties of the hemicellulose decrease with the time. The pulp strength of the pulp fibre will for this reason be only slightly increased in several cooking processes. It has, surprisingly, become apparent that what is desired to a larger degree is only to obtain that part of the dissolved hemicellulose that has the longest chains, and it is this fraction of the hemicellulose that precipitates first. If the time in certain cooking systems becomes too long, also those fractions of the hemicellulose with short chain lengths will precipitate, while at the same time the longer chains of hemicellulose that already have been re-precipitated will be broken down.
Another variant for the influence of the precipitation of hemicellulose onto the fibre is revealed in EP,B,1.115.943. In this variant, cooking fluid that is rich in hemicellulose is withdrawn early in the cook and this cooking fluid with a high content of dissolved hemicellulose is returned to the final phase of the cook. A substantial retention time in the final phase of the cook, greater than 60 minutes, is required in order for the precipitation process to be given sufficient time to be activated. This type of cooking process is shown schematically in FIG. 1, “Xylan”, where it is shown that cooking fluid with a high content of hemicellulose is withdrawn early (the second strainer section from the top), and returned to the digester in a cooker flow (the fourth strainer section from the top), where a certain volume of spent cooking fluid can at the same time be withdrawn. This cooking fluid with a high content of hemicellulose can in this manner be reintroduced into the cook, in order to be present during the final phase of the cook, with a duration of at least 60 minutes.
A first aim is to offer an invention that fully or partially solves the disadvantages and problems described above, and to be able effectively to reduce the retention time of the cooking fluid through the complete cook in systems with a far too long retention time of the liquor in the digestion system. The cooking fluid is to be present with the chips in the cooking vessel as long as possible, but the retention time is to be reduced as far as possible.
The principle of the invention is that the level of dissolved hemicellulose is maintained in the cooking fluid throughout the cook, which hemicellulose is dissolved very early in the cook, typically within the first 20-30 minutes of the cook. The process for re-precipitation of hemicellulose requires a long retention time in order to note a measurable effect in raising the yield, typically a retention time of at least 50-70 minutes is required.
A second aim of the invention is to offer a method and an arrangement for continuous cooking that gives a cellulose pulp with optimised and improved pulp quality with respect to the tensile strength, tear strength and beatability of the pulp fibre.
It has proved to be the case that the yield and the strength of the pulp increase with increasing early precipitation of the hemicellulose onto the fibre, but the longer hemicellulose chains are broken down with longer retention times and the strength of the pulp decreases.
A third aim is to maintain the dissolved hemicellulose in the cook and to ensure that it remains right up until the final 15 minutes of the cook, in order to ensure that it has sufficient time to re-precipitate onto the pulp fibre.
A fourth aim of the invention is to reduce specifically the H factor of the cooking fluid, i.e. the time that the cooking fluid is held at the cooking temperature. This means that it is possible actively to control the retention time of the hemicellulose that has been released from the chips in the cook such that it does not have sufficient time to be broken down: it can instead be influenced in a controlled manner such that the original form and structure of the hemicellulose are not changed as a result of breakdown, and it can be precipitated onto the pulp fibre in this form.
A fifth aim is to have a high fluid/wood ratio throughout the complete cook. This entails several advantages since the alkali concentration can be held more even during the cook since a greater amount of kilograms of alkali per kilogram of chips can be established in the cooking fluid, which ensures that the alkali concentration does not fall so greatly during the cooking process as the alkali is consumed as the wood is delignified.
A sixth aim is to implement in modern continuous digesters that have production capacities in the range 2,000-3,000 tonnes, or greater, of pulp per day, where these digestion plants consist of digesters with diameters that easily exceed 6-8 meters, a completely new cooking concept that adopts extremely large strainer sections at the end of the digester where very large volumes of spent cooking fluid are withdrawn, or from the point of view of control, it is attempted to withdraw such volumes. This cooking technique is totally different from other modern cooking techniques for large digestion plants in which several withdrawal positions in the cook are available for several different purposes, and the cook in this way loses the hemicellulose that has been dissolved in the cooking fluid before the final phases of the cook. One aim of the plurality of withdrawals is to maintain low levels of the dissolved DOMs (including hemicellulose) during the cook, but this unavoidably gives losses of the dissolved hemicellulose. Another aim is that limitations have been seen in the withdrawal of cooking fluid from the cook in these large digesters and it is therefore necessary to use several withdrawal positions, and this also removes dissolved hemicellulose from the cook before the final phases of the cook.
A seventh aim is to make it possible to reduce the highest alkali concentration during the cook, typically that which is established at the beginning of the cook, while at the same time retaining a relatively high and even alkali concentration during the complete cook, until the final phases of the cook. The time during which the alkali in the cooking fluid is consumed is reduced through the establishment of a high fluid/wood ratio in the cook and the reduction of the retention time of the cooking fluid in the cook, under the condition that the chips have a pre-determined retention time. If, for example, one and the same alkali charge is used at the beginning of the cook while the retention time of the cooking fluid is reduced, the level of residual alkali in the black liquor withdrawn will increase as a result of the reduction in reaction time. This can be exploited through instead reducing the alkali charge at the beginning of the cook with the aim of maintaining the same level of residual alkali in the black liquor. The high fluid/wood ratio and the reduction in retention time of the cooking fluid work together to reduce the alkali concentration at the beginning of the cook, under the condition that it is still possible to ensure a given level of residual alkali in the withdrawn black liquor and an effective alkali concentration during the complete cook. This allows a better pulp strength since it is known that alkali concentrations during the cook that are too high can have an adverse influence on the strength of the pulp.
The aims described above are achieved with a method in accordance with the present invention.
The suggested invention concerns a method and an arrangement that, in combination with continuous cooking of chemical cellulose pulp, is to give a cellulose pulp fibre with high tensile durability, tear strength and beatability.
The strong pulp fibre is achieved through having a maintained high fluid/wood ration throughout the complete cook with essentially the same cooking fluid at the end of the cook as at its beginning. The retention time for the cooking fluid in the digester is in this way reduced, and thus also the H factor of the cooking fluid. The total amount of dissolved hemicellulose is in this way reduced, which hemicellulose precipitates back onto the cellulose pulp fibre and gives the fibre its strength-enhancing properties. However, since the strength-enhancing properties of the hemicellulose are highest at the beginning of the cook and become less with time, the chips will obtain a higher tensile strength, tear strength and beatability than what would have occurred in a cooking process with a higher H factor and longer retention time of the cooking fluid. It is, however, necessary that the major part of the cooking fluid is retained throughout the complete passage through the cook, such that as much as possible of the strength-enhancing properties from the hemicellulose have sufficient time to precipitate out onto the fibre.
Further characteristics and aspects of the invention, and its advantages, are made clear by the attached patent claims and by the detailed description of some embodiments given below.